Three different processes of producing nodular cast iron are known in the art. The molten material is treated with cerium misch metal in one of the known processes, with magnesium in another and with calcium in a further process.
From U.S. Pat. No. 2,837,422 it is also known to treat molten cast iron with master alloys which, in addition to magnesium, contain rare earth metals. All cast iron products thus treated have improved mechanical properties and the treatment with magnesium is particularly desirable. A formation of nodular graphite in hypereutectoid and/or hypereutectoid melts will be achieved by a treatment with Mg and only in hypereutectoid melts by a treatment with cerium misch metal. Calcium dissolves only very slowly in cast iron (British Patent Specification No. 718,177).
It is known that the treatment of molten cast iron with magnesium will result in reactions of the magnesium with sulfur and oxygen with formation of sulfides and oxides of magnesium as reaction products, which form inclusions in the castings and lower their quality and cause scarred and unsound surfaces to be left on the castings. For this reason, it is also known to effect a desulfurization by a first magnesium treatment of the molten cast iron and after a reheating to effect a second treatment with magnesium to form nodular graphite.
However, since removal of the reaction products does not occur after the first treatment, the castings obtained from such melts have inclusions and surface defects ("Giesserei" 40, 1953, pages 93-103).
In the process disclosed in German Publication No. 21 43 521, a treatment with magnesium in two stages is effected and slag is removed after the first pretreatment. But, in that case, a reheating of the molten cast iron between the first and second treatments is required and must be tolerated.
In order to eliminate or avoid these disadvantages, it is known to use ceramic filters having an open-cell foam structure in the sprue system of a casting mold and to filter the molten metal. This treating step, however, significantly increases the costs of making the castings (European Patent Publication No. 0 126 847).
From Published Japanese Patent Application No. 61/15,910, it is known to treat high-sulfur cast iron with additives in two stages. An additive which contains rare earth metal is added in the first stage and a magnesium-containing additive in the second. The two additives may, alternatively, be added at the same time.
While the simultaneous addition of magnesium and rare earth metals is a simple and common process step, the rare earth metals thus added considerably contribute to the formation of reaction product slag (AFS Cast Metals Res. J., Sept., 1970, pp. 135/136).
It is also known to add mischmetal to a molten cast iron which has been treated with magnesium in order to avoid a slagging of the molten material by sulfur and oxygen and to oppose the disturbing effects of Ti, Pb, Sb, Bi, Al, Cu, As, Sn on the formation of nodular graphite ("Modern Casting", June 1969, pp. 94/95).
A slagging by sulfur may take place by a reoxidation of MgS with oxygen, which enters the system from the ambient atmosphere or in chemically unstable compounds by the reactions EQU 2MgS+O.sub.2 .fwdarw.2MgO+2S EQU 2S+2Mg.fwdarw.2MgS
As a result, sulfur is redissolved and causes the structural growth to degenerate. That reaction sequence is dependent on temperature and time and is described as "fading".
The progress of the above-mentioned reactions will be particularly promoted by turbulence occurring in the mold cavity during the casting operation. A particularly undesirable effect is produced by magnesium oxide slags appearing as segregation products in thick-walled castings, because such slags will result in abnormal structures by which the dynamic properties of the casting are seriously diminished.
Particularly, if the molten material to be cast has controlled low residual magnesium contents, the reaction mentioned above will result in a so-called resulfurization of the magnesium-treated iron after a few minutes so that the nodular graphite that has been found will be transformed to vermicular or flaky graphite and, therefore, the metallurgical goal of nodular graphite will be missed. In most cases, the temperature will have decreased so much that additional magnesium cannot be added and such addition would again result in a formation of detrimental reaction products of the known kind.